Although current hearing devices improve listening in quiet environments, they do little to improve speech understanding in noisy backgrounds. The long-term goal of this research program is to fully understand the relationship between auditory perception and physiological mechanisms responsible for adapting to the local soundscape. The objective of this particular application is to understand the role of the medial olivocochlear (MOC) reflex in the perception of fluctuating sounds and on speech-in-noise performance in normal hearing and hearing impaired listeners using perceptual, electrophysiological, and auditory modeling techniques. The central hypothesis of the proposed research is that cochlear hearing loss limits the ability of the MOC reflex to regulate cochlear gain, thus preventing the putative perceptual and neural benefits associated with the reflex. The rationale of the proposed research is that a detailed description of the influence of the MOC reflex on human auditory function has the potential to translate to a better understanding of why hearing devices provide little benefit to improving speech-in-noise performance in hearing impaired adults. This detailed description will be obtained by completing the following specific aims: 1) Determine the role of the MOC reflex in the detection of temporal fluctuations and the identification of speech syllables in noise; 2) Determine the effect of eliciting the ipsilateral MOC reflex on the compound action potential (CAP) in human subjects with and without cochlear hearing loss; and 3) Simulate the influence of the MOC reflex on auditory function in listeners with and without cochlear hearing loss. This approach is innovative because it links empirical findings from normal and impaired perception and electrophysiology to a quantitative theoretical framework using a computational auditory model. The proposed research is significant because it is expected to lead to 1) a clear understanding of the role of the MOC reflex in humans with normal and impaired auditory systems, 2) effective procedures for evaluating the influence of the MOC reflex on auditory function, 3) novel MOC-inspired signal processing algorithms for hearing devices, and 4) a better quality of life for hearing impaired individuals by providing improved communication with family, friends, and co-workers. To achieve the research objectives of this application, the principal investigator (PI) has designed a career development plan to acquire expertise in speech perception and electrophysiological methods, and deepen expertise in computational auditory modeling. Training in these areas will be obtained by frequent communication and planned mentoring activities with senior scientists in the PI's mentoring/advising team. This training will provide the PI with the skills needed to become an independent investigator, and prepare for a career of research productivity.